Signal processing arrangement



1959 J. P. cosTAs 2,868,

SIGNAL PROCESSING ARRANGEMENT Filed March 15, 1957 v 2 Sheets-Sheet 1 a 4 FIG.|. e 1 0 I l CRYSTAL 5 HARMONIC TUNED E AMPLITUDE I CONTROLLED MIXER OSCILLATOR GENERATOR AMPLIFIER DETECTOR fo,fc I 1 ,1, CONTROLLED OUTPUT OSCILLATOR CIRCUIT l2 REACTANCE TUBE CIRCUIT T F|G.2a.

U fb u 20: 9g fa fc "u g |Aw---Aw-| EREouE-cY- F|G.2b.

D II. :I O

INVENTORI JOHN P. COSTASI HIS ATTORNEY.

of different frequencies.

United States 2368,? Patented Jan. 13, 1959 2,868,981 SIGNAL PROCESSING ARRANGEMENT John P. Costas,

Electric Company, a corporation of New York Application March 15,1957,SerialNo.646,43tl

10 Claims (Cl.250-36) My invention relates to signal processing arrangements and particularly to methods and means for providing a large number of signals of high frequency stability.

arrangements have proven expensive to construct.

reference frequency source.

It is a further object of my invention to provide a con tinuously tunable source of signals of high frequency stability and accuracy.

In accordance with one embodiment of my invention, means are provided for deriving, from a single stable reference frequency source, at least two signals of difierent frequency fl, and f such as may be derivable from a harmonic series having a fundamental frequency as the reference frequency, generating a signal having a frequency f intermediate f and f comparing the signals corresponding to f f and f to derive a control signal, and using the control signal to maintain the frequency f at a predetermined frequency with respect to the signals f and f The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention, itself, however,

both as to its organization and method of operation, together with further objects and advantages thereof, may

best be understood when taken in connection with the accompanying drawings in which Fig. 1 is a block diagram of one embodiment of my invention, Figs. 2a and 2b, 3a, 3b and 5 illustrate graphically certain waveforms useful in explaining the operation of the present invention, and Fig. 4 illustrates in block diagram form a further embodiment of the present invention.

Referring to Fig. 1, there is shown a method and means for obtaining signals of many different frequencies of crystal stability while employing a tunable oscillator in conjunction with an oscillator having a single crystal control. It is desired to provide over lead 1, for application to an output circuit 2, signals of a frequency stability comparable to that available at the output of the crystal oscillator or reference signal source 3 but In accordance with one an- ,bcdirnent of the invention, two signals of different fre Fayetteville, N. Y., assignor to General 7 then be given as quency are derived from a reference signal source in such a manner as to substantially maintain the frequency stability of the reference signal. In accordance with one embodiment, source 3 supplies a reference signal of substantially constant amplitude to a harmonic generator 4. Harmonic generator 4 operates in response to the supplied reference signal to deliver at its output lead 5, a plurality of harmonically related signals, as for eX- ample, of frequency f, and f shown in Fig. 2a. In Fig. 2a, frequency is plotted as abscissa and signal occurrence is plotted as ordinate. The frequency spacing between the frequencies f and f and between f and f is shown to be equal.

Oscillator 6 a frequency range corresponding. to that desired at the output circuit 2. The oscillator signals available from the source 6 and the harmonically generated signals available from source 4 are mixed in mixer circuit 8 to provide at the output lead 9 a amplitude varies in accordance with the relative phases of the signals available over leads 5 and 7. The mixer circuit 8 may comprise any of the well-known circuits which beat together or multiply input signals to provide a resultant output signal. The specific characteristics of the signals available over lead 9 and how they are employed to control the operation of oscillator 6 will be described in detail later. For purposes of our discussion, assuming the frequency, f of the oscillator 6 was selected to be half-way between that of the signals f and i as shown in Fig. 2a, the signals available over lead 9 would have a beat frequency f,.. These signals, upon being amplified to a suitable level in amplifier 10, are detected or converted to a direct signal by the amplitude detector lll and applied over lead 12 to a frequency control circuit 13. Frequency control circuit 13 operates to adjust the frequency of oscillator 6 in a desired direction. The frequency control circuit 13 may comprise one of the many well-known varieties available in the art, and in particular may comprise a reactance tube type frequency control circuit.

A previously mentioned, the mixer circuit 8 has supplied to it a plurality of harmonically related signals, as shown in Fig. 2a, which have been derived from the crystal controlled oscillator 3. The frequency of oscillator ti was selected to be somewhere near the midpoint between the crystal controlled frequencies f,, and f Under these circumstances the mixer provided at its output a resultant signal of frequency 1, (equal to derived from the products f -f and 12-13,) whose amplitude varied as the phase of the signal available from oscillator 6 varies with respect to the phase of the signals i and f This may be represented mathematically as follows. Fig. 2a represents a pair of harmonics with frequencies f and f The tunable. oscillator frequency f is shown at a point midway betwen 1 and f Let the Where t is time and p (M, and angles of the three are the fixed phase signals. The output of mixer 8 will provides signals at its output lead 7 within low frequency signal whose 3 which results in C b-* a) +ba cos (we b) +c b when difference frequency terms only are considered and Where W =27rf W =21rf and W =21rf Now, if the frequency of oscillator 6 lies exactly midway between w and w,

where Aw represents one half the between harmonics h (t) and h (t). then be rewritten as Manipulating Equation 7 by use of trigonometric identitiesyields Equation 5 may frequency difference oscillator 6, in accordance with the characteristic shown in Fig. 2b. If the reactance circuit 13 is adjusted to operate on the XAZ portion of the characteristic curve of Fig. 2b, and in particular about the dotted reference line 0, desired phase control of oscillator 6 can be achieved. For example, if the phase of oscillator 6 should increase in the direction A to Z, a control signal is generated by the reactance tube which returns the phase of oscillator 6 to its desired level. On the other hand, :if the phase of oscillator the direction A to X, reactance tube provides an appropriate control signal which adjusts the phase of oscillator 6 back to its desired level. Thus the arrangement disclosed will operate to maintain the frequency of oscillator 6 midway between the frequencies f and f Since the frequencies and f were derived from the crystal control oscillator 3, their stability corresponds to the stability of the crystal controlled oscillator. Correspondingly, the frequency of the oscillator 6 will be regulated to a comparable stability.

While I have described the frequency f as having been selected to occur somewhere between the midpoint of the most adjacent hormonically related signals f,, and f it is obvious that the invention can be practised in the case where thefrequency f is made to occur somewhere near the midpoint of other pairs of sidebands, as for example as shown in Figs. 3a and 3b where frequency is plotted as ordinateand signal occurrence is plotted as abscissa. In Fig. 3a, the frequency f may be compared with the most adjacent sidebands (N+1)f and (N-\-2)f where'N is any integer and f is the crystal reference frequency, or with other sideband pairs such as Nf and (N+3)f Similarly, the f can be selected to coincide with one'of the harmonics, as for example (N+3)f shown in Fig. 3b. Again the frequency h, in this latter case can then be compared with the most adjacent harmonies or furtherremoved harmonics, as for example -l-Ufo and -l )fo- ,The present invention may also be employed to provide a continuously variable signal of high frequency stability and accuracy. Referring to Fig. 4, a portion of-gthecircuit labeled L may be identical to that. of Fig. 1 previously described and accordingly the common reference numerals have been retained in Fig. 4. Thus at the output lead 1 there is provided any one of a large 6 should decrease in number of signals of discrete frequency having a stability comparable to that of the crystal oscillator circuit 3. The second portion of the circuit of Fig. 4, labeled M, is adapted to utilize the signal available over lead 1 in order to provide at the output lead 15 a continuously variable frequency signal having a frequency stability comparable to that of thecrystal oscillator 3. The circuit may also be adapted. to provide one of a large number of signals of discrete frequency having a frequencystability comparable to that ofthe ioscillator. 3 but occurring in frequency intervals which are smaller than the signals available over lead 1 from circuit L. These signals may then be applied to an output circuit 16 for desired utilization. To accomplish these desired results, the oscillator 17 is selected to have a frequency corresponding to that of f shown in Fig. 5a. In Fig. 5 frequency is plotted as ordinate and signal occurrence is plotted as abscissa. In order to maintain the frequeney of oscillator 17 at the desired frequency level, the signal of frequency f is mixed with thesignal f available over lead 1 and the beat signal or difference between these two is applied over leadv 19 to a phase-detector 20. The phase detector is also supplied with .a signal i available from the oscillator 21 over lead, 22. Referring to Fig 5 again, it is noted that the frequency f has a predetermined relationship with respect to that of the signals f and i namely the difference frequency therebetween. It is obvious that f can .be either above or below f.,. The resultant output of the phase detector 2t) is a direct signal whose amplitude and polarity is representative, of the'diiferencel in phase between the signals available on lead 19 from the mixer 18 and the signal available on lead 22. from the oscillator 21. The resultant or control signal is applied to a frequency controlcircuit 23 over lead 23a which in turn adjuststhe frequency of oscillator '17 in the proper directionto maintain its frequency at the desired frequency level i of the crystal oscillator As previously explained, the frequencycontrol circuit 23 maycomprise a reactancetube type control circuit. The result is that at output lead 15 there is;pr.ovided signals of frequency f which have very nearly the'stability 3 but which may be continuously varied in frequency in accordance with ,thefrequency selected and made available by oscillator 21.

If the switch24 is moved to position 25, the oscillator 21 is replaced with the discretely tuned oscillator :26 which delivers at its outputelead any one of a number of discrete frequencies. In a preferred embodiment-the oscillator 26 comprisesa circuit arrangement, similarxto' that shown in Fig. 1 and correspondingto 'theqcircuit portion:L ofFig. 4. The :steps offrequencyavailable from source 26, of course, are made to be finer thanithat available on lead 1 from the circuit portion L of Fig. 4.

While aspecific embodiment has=been shown and described, it will of course be understood thatvariousmodifications may yet be devised: by thoseskilled in the art which will embody the principles .of the invention-andbe found in the true. spirit and scope thereof.

What I claim and desire. to secure by Letters Patent of the United States is:

1. In combination, a source of .a signal of highly, stabilized frequency 1, means responsive to said ffrequency signal for providing at leastto signals 1, and f .of sub stantially the-same high stability'but of different frequency, a controllable source of a signal of frequencyzf means formultiplying said signals of frequency '3, and) with said signal of frequency 'f to 'derive'a resultant signal of frequency dependent upon the 1-mult-iplication products f f and f -f means for amplitude detecting said ,-resultant signal, and means responsive to said amplitude detected resultantqsignal for controlling-{the frequency-f of said signal*from said controllable oscillator to be frequency,

, quency f of signals signal of frequency 7,

frequency f and f derive a first mixed signal of frequency a controllable source of signal of frequency f means f and f respectively,

2. In combination, a source of a signal of highly stabilized frequency f, means for providingat leasttwo harmonically related'signals f stlnd f5 in response to said 7 frequency signal, a controllable source of a signal of frequency f means for mixing said signals of frequency f,, and f with said signal of frequencyj to derive a mixed signal, means for amplitude detecting said mixedsignal, and means responsive to said amplitude detected mixed signal for controlling the frequency f of said signal from said controllable oscillator to be 3. In combination, a source of a signal of highly stabilized frequency f, means responsive to said signal of frequency f for providing at least two signals and f of substantially the same high stability but of different a controllable source of a signal of frequency f means for multiplying said signals of frequency f and 7",, with said signal of frequency f to derive a first resultant signal of frequency dependent upon the products f ,-f and f f means for amplitude detecting said first resultant signal, and means responsive to said amplitude detected resultant signal for controlling the frefrom said controllable oscillator to be fa+fc f means responsive to the relative phase of said signal of frequency f and said second resultant signal for providing a third resultant signal, andmeans responsive to said third resultant signal for controlling the frequency f of said signal from said second controllable source to be f if 4. In combination, a source of a signal of highly stabilized' frequency 1, means for providing at least two harmonically related signals 1,

a first controllable source of a sigmeans for mixing said signals of with said signal of frequency f to signal, means for amplitude detecting said first mixed signal, means responsive to said amplitude detected first mixed signal for controlling the frequency f of signals from said controllable oscillator to be fa+fc 2 nal of frequency f a source of a second controllable signal of frequency fa, means for mixing said signal of frequency f with said to derive a second mixed signal,

responsive to said signal of frequency i and said second and in response to said a source of a second controllable signal of frequency f means responsive to the relative frequency of said signal frequency f and said signal of frequency f to derive a second resultant signal, a controllable source of signal of frequency f means responsive to the relative phase of said signal of frequency f, and said second resultant signal for providing a third resultant signal, and means responsive to said third resultant signal for controlling the frequency of said signal from said second controllable source to be f if 6. In combination, first means comprising a source of a signal, of highly stabilized frequency 7, means for deriving from said signal of freqency f at least two signals 7",, and f of substantially the samehigh frequency stability, a controllable source of a signal of frequency f means for multiplying said signals of frequency f,, and f with said signal of frequency f to derive a multiplied signal, means responsive to said multiplied signal for maintaining the frequency f of signals from said controllable source equal to fa+fc 2 mixed signal for providing a resultant signal having an amplitude and a polarity which is a function of the I relativephases of said second mixed signal and said signal of frequency i sultant signal for controlling the frequency i of said and means responsive to said resignal from said second controllable source to be f if 5. In combination, a source of a signal of highly stabilized frequency f, means responsive to said signal of frequency f for providing at least two signals of frequency a controllable source of a signal of frequency f means responsive to the relative frequency of said signals of frequency f,,, f and f to derive a first resultant signal, means responsive to said first resultant signal for controlling the frequency f of said signal from said controllable oscillator to be quency f, and f with trollable source of signals of frequency f and f,,, means for multiplying said signals of frequency ;f;,, f and f to derive a second multiplied signal, and means responsive to said second multiplied signal for maintaining the frequency f of signals from said second source equal to the difference between the frequencies of signals of frequency f and f 7 In combination, a source of a first and second signal of frequency f and f respectively, means for deriving a third signal of frequency f from said first and second signal wherein I a fourth signal of frequency f,,, signal of frequency f wherein the frequency means for deriving a fifth from said fourth and third signals 5 equals the difference between the frequencies f and i 8. In combination, a source of a signal of highly stabilized frequency f, means for deriving from said signal at least two signals f and f of substantially the same high frequency stability, a source of a signal of frequency j means for multiplying said signals of fresaid signal of frequency f to derive a multiplied signal, and means responsive to said amplitude detected multiplied signal for maintaining the frequency f of signals from said controllable oscillator equal to fll+f0 2 9. In combination, a source of a first, a second and a third signal offrequency f f and f respectively, means responsive to the relative frequencies of said first, second and third signals for deriving a control signal, and means responsive to said control signal for maintaining the frequency f of said source equal to l fa+fc t 10. In combination, a source of a first, a second and a third signal of frequency f f and f respectively, means responsive to the relative frequencies of said first, second and third signals for deriving a control signal, and means responsive to said control signal for maintaining the frequency f of said source equal to a source of a fourth and fifth signal of frequency i and 7",, respectively, means responsive to the relative frequencies of said second, fourth and fifth signals for I "Referencefcitedin flne file bf this patent deriving 'a' further control signal, and means responsive STATES PATENTS H to said further control signal for maintaining the fre- 2,398,694: C se 1; .t" Apt ,6

quency f of said last-named seurce at a value such that .v t .7 t 3 .i v v t the frequency f is the difference frequency between said 5 3 86 Buick et a -i'ff" k -I2 fourth'and second signals. 2,662,181: I fl g 3; .9 

